The present invention relates to a heat conductive rubber composition material and a heat conductive rubber sheet which are used to secure a higher thermal connection between a heat sink for cooling and electric parts to be cooled which are arranged in various electronic and electric instruments.
The cooling of the heat generating electronic parts such as semiconductor devices arranged in various electronic and electric instruments like computers has been considered as the important technical object to be attained. Conventional methods for cooling semiconductor devices and the like include a fan fixed to the housing of the electronic instrument, in which the semiconductors are arranged. The fan draws cooler air into the housing, which consequently removes heat from a heat sink fixed to the semiconductor devices. As the heat sink is cooled, so too will be the semiconductor.
Where a heat sink is fixed to the semiconductor devices or the like which device is intended to be cooled (hereinafter referred to as xe2x80x9cpart to be cooledxe2x80x9d), the thermal connection between the part to be cooled and the heat sink is important. When the thermal connection is low between the part to be cooled and the heat sink, sufficient cooling cannot be obtained. For example, when the part to be cooled is placed in direct contact with the heat sink, the contact resistance in the interface therebetween is, in general, so large that the part to be cooled is not effectively cooled, thus failing to sufficiently cool the part to be cooled.
When the part to be cooled is soldered to the heat sink, the thermal resistance is remarkably lowered in comparison with the case in which the part to be cooled is directly contacted by the heat sink. However, even if the part to be cooled is soldered to the heat sink, when the difference in thermal expansion between the part to be cooled and the heat sink is large, the differing rates and magnitude of expansion can deleteriously affect the effectiveness of the heat sink. For example, where an aluminum sheet which is excellent in thermal conductivity, is applied to the material for the heat sink, they together have a large thermal expansion. The semiconductor device, on the other hand, as the part to be cooled, has a small thermal expansion. Thus, the generation of heat, stress is created in the soldered portion between the heat sink and the part to be cooled, causing a bending or peeling off of the soldered portion.
One prior art method to solve the above problem is to interpose grease, for example silicone grease or silicone rubber, between the part to be cooled and the heat sink. More specifically, when the grease or silicone rubber is interposed therebetween, it is expected that the grease or silicone rubber will absorb the difference in the thermal expansion between the part to be cooled and the heat sink so as to suppress the bending or peeling off. In addition, it is expected that the grease or silicone rubber will fill up small gaps existing between the parts to be cooled and the heat sink so as to lower the thermal resistance in the interface therebetween.
Since however, the grease or silicone rubber to be interposed between the part to be cooled and the heat sink has a higher thermal resistance in general, it is not easy to thermally connect the part to be cooled and the heat sink using this method. In addition, since the substance such as silicone grease has a remarkably lower thermal conductivity than the metal material forming the heat sink, it is not easy to further improve the cooling efficiency. Therefore, the art is in need of means for lowering the contact resistance between the part to be cooled and the cooling body while avoiding the above noted drawbacks.
Japanese Patent Publication No. 57-100148 discloses a method of interposing between the part to be cooled and the heat sink, a silicone rubber composite material which is prepared by mixing a filler such as aluminum oxide particles, aluminum nitride particles or boron nitride particles, which have relatively higher thermal conductivity, into a silicone rubber (hereinafter referred to as xe2x80x9crubber composition materialxe2x80x9d).
The rubber composition material of the silicone rubber with the aluminum oxide particles, boron nitride or the like mixed therein has a higher thermal conductivity than the usual silicone rubber. When the thus prepared rubber composition material is interposed between the part to be cooled and the heat sink, it can be expected, to some degree, that the difference in thermal expansion therebetween is absorbed by the above rubber composition material. More specifically, it is expected that the bending or peeling off caused by the remarkable difference in the thermal expansion between the semiconductor and heat sink is to be prevented from occurring. In addition, errors in the size of the parts and errors in fabric are expected to be absorbed by the above rubber composition material.
In general, in order to improve the thermal conductivity of the above rubber composition material in which the aluminum oxide particles, boron nitride or the like as the filler are mixed into the silicone rubber, it is necessary to increase the amount of the filler. When the amount of the aluminum oxide particles, boron nitride or the like as the filler increases, however, the prepared rubber composition material becomes harder. Thus, the effect of absorbing the difference in thermal expansion between the part to be cooled and the heat sink, as well as the effect of absorbing the errors in the size of the parts and errors in fabric can not be expected.
Although the cost of aluminum oxide particles as the filler mixed into the silicone rubber is not expensive, making the method economically attractive, the prepared rubber composition material, as based above, has a tendency to become harder when the mixing amount thereof increases. Aluminum nitride particles or boron nitride particles are economically unfavorable for practical use due simply to a relatively high material cost. As is clear from the forgoing, the art is still in need of a method and composition capable of providing a high thermal connection between the part to be cooled and the heat sink in order to effectively cool the part to be cooled.
With electronic and electric instruments progressing to provide higher efficiency and smaller overall dimensions, the amount of heat and density of the areas over which it is radiated are increasing. In addition, the popular reduced dimension of electronic and electric instruments requires that the space used for heat sinks be narrowed. Therefore, it is desired to realize the means to obtain much higher thermal connection between the parts to be cooled and the heat sink in order to improve the efficiency of cooling so that an acceptable degree of cooling can be obtained by a smaller heat sink or that more cooling can be obtained for a longer heat sink.
The object of the present invention is therefore to provide a heat-conductive rubber composition material and a heat-conductive rubber sheet to realize a higher thermal connection between the parts to be cooled and the heat sink.
In order to attain the above object, there is provided a heat-conductive rubber composition material, which comprises a rubber essentially consisting of an acrylic rubber, a butyl rubber, an ethylene propylene rubber or a blended rubber in which at least two of said acrylic rubber, said butyl rubber, and said ethylene propylene rubber are blended, and 50 to 85 weight % of hydrophobic magnesium oxide particles.
Furthermore, there is also provided a heat-conductive rubber composition material, wherein said hydrophobic magnesium oxide particles comprise magnesium oxide particles obtained by burning at a temperature from 1100 to 1600 degrees C magnesium hydroxide having a high dispersion with an average article size of up to 2 xcexcm, and having 1 to 20 m2/g of BET specific surface area, then grinding the burned magnesium oxide to prepare magnesium oxide particles having an average secondary particle size of up to 20 xcexcm.
Furthermore, there is also provided a heat-conductive rubber composition material, wherein said rubber comprises said acrylic rubber.
Furthermore, there is also provided a heat-conductive rubber composition material, which contains 1 to 10 weight % of magnesium silicate clay material containing water.
Furthermore, there is also provided a heat-conductive rubber composition material, which contains 2 to 20 weight % of organic compound having a melting point of 40 to 90 degrees C.
There is provided a heat-conductive rubber sheet prepared by forming the above heat-conductive rubber composition material to a sheet type.
Furthermore, there is also provided a heat-conductive rubber sheet prepared by coating the above heat-conductive rubber composition material onto both surfaces of a metal sheet.
Furthermore, there is also provided a heat-conductive rubber sheet, wherein an adhesive agent is further coated on at least one surface thereof.
There is provided a heat-conductive sheet, which comprises a soft metal sheet and a rubber layer formed on at least one surface of said soft metal sheet.
Furthermore, there is also provided a heat-conductive sheet, wherein said soft metal sheet comprises a material having tensile strength of up to 100 N/mm2.
Furthermore, there is also provided a heat-conductive sheet, wherein said soft metal material comprises aluminum or copper.
Furthermore, there is also provided a heat-conductive sheet, wherein said soft metal material comprises pure aluminum having at least 99.99% of purity.
Furthermore, there is also provided a heat-conductive sheet, wherein said rubber layer comprises a rubber composition material comprising a mixture of a rubber essentially consisting of an acrylic rubber, a butyl rubber, an ethylene propylene rubber or a blended rubber blending at least two of said acrylic rubber, said butyl rubber, and said ethylene propylene rubber, and 50 to 85 weight % of hydrophobic magnesium oxide powder.
Furthermore, there is also provided a heat-conductive sheet, wherein said rubber composition material further contains 1 to 10 weight % of magnesium silicate clay material containing water.
Furthermore, there is also provided a heat-conductive sheet, wherein said rubber composition material further contains 2 to 20 weight % of organic compound having a melting point of 40 to 90 degrees C.
Furthermore, there is also provided a heat-conductive sheet, wherein an adhesive agent is coated on at least one surface of said rubber layer.